The objective of the present study was to evaluate the risk factors and outcomes associated with hyponatremia in patients with Guillain-Barré syndrome (GBS). We retrospectively studied 80 consecutive patients with GBS who visited our hospital and compared clinical, laboratory, and electrophysiological findings of patients with and without hyponatremia. Disability was evaluated using the Hughes grading system. Of the 80 patients, 18 (23%) had hyponatremia. Hyponatremia was significantly associated with older age (P = 0.003), urinary retention (P < 0.0001), Hughes grade ≥ 4 at admission and nadir (P = 0.003 and P < 0.001, respectively), acute inflammatory demyelinating polyneuropathy subtype (P = 0.017), sepsis (P = 0.001), mechanical ventilator support (P = 0.013), longer hospitalization length of stay (P < 0.0001), and inability to walk independently at 6 months (P < 0.001). Multivariate analysis performed to assess the risk factors of hyponatremia revealed that urinary retention (odds ratio [OR] 30.7, 95% confidence interval [CI] 3.6-264.4; P = 0.002) and mechanical ventilator support (OR 13.8, 95% CI 1.6-118.0; P = 0.017) were significant independent risk factors of hyponatremia. In assessing the outcomes of patients with hyponatremia, multivariate analysis showed that hyponatremia was independently associated with hospitalization length of stay ≥ 60 days and inability to walk independently at 6 month, with the former showing statistical significance but the latter not (OR 9.3, 95% CI 1.8-47.7; P = 0.007 and OR 4.9, 95% CI 0.9-26.3; P = 0.066, respectively). Therefore, we demonstrate that, along with mechanical ventilator support, urinary retention-possibly indicating autonomic dysfunction-is a risk factor of hyponatremia in GBS. Moreover, we confirm that hyponatremia is associated with poor outcome in GBS.